The primary circuit of a nuclear reactor in the course of operation contains water containing boric acid at a pressure in the vicinity of 155 bars which serves at the same time for the cooling of the core of the reactor located in the tank and for controlling the reactivity of this core.
The primary circuit of the reactor in which the steam generators are arranged also serves for the transfer of heat from the core of the reactor to the steam generators where vaporization of the feed water takes place by heat exchange with the primary water.
If the primary circuit shows a leak, it is necessary to compensate for the latter by injection of additional water into this primary circuit. In the case of a very large leak and, for example, of a real break in a pipe of the primary circuit, it is necessary to send very large amounts of coolant liquid constituted by the water supplemented with boric acid, in a very short time, to avoid a very considerable temperature rise of the core which could lead to melting of the latter.
In the case of a slight leak accompanied by very slight depressurization of the primary circuit, the cooling water is injected at a pressure higher than the normal pressure of the primary circuit, by means of a special circuit called a volumetric control circuit situated outside the safety enclosure in which the vessel of the reactor is located.
In the case of bigger leakages, it becomes necessary to place in operation a device for the emergency cooling of the reactor by the injection of cooling liquid, called the safety injection system.
Such a safety injection system generally includes a group of accumulators containing cooling liquid under a pressure in the vicinity of 40 bars situated inside the safety enclosure of the reactor and two independent injection systems situated outside the safety enclosure.
The accumulators containing cooling water at 40 bars, generally called medium pressure accumulators, are placed in communication with the cold branches or legs of each of the loops of the primary circuit with the interposition of a valve whose opening occurs at the moment when the pressure in the primary circuit drops below the pressure of the accumulator. These accumulators are constituted by a water reserve in temperature equilibrium with the inside of the safety enclosure and above which a gas such as compressed nitrogen maintains a pressure a little higher than 40 bars.
The injection means arranged outside the safety enclosure include at least one double circuit equipped with high or medium pressure pumps with at least one double circuit equipped with low pressure pumps fed the one and the other with cooling water through the boric acid storage tank of the pools of the reactor.
The rated operating pressure of the high pressure or medium pressure pumps equiping the one or more injection circuits is close to 100 bars in the case of a nuclear reactor with four loops of 1300 MW power, whilst the rated operating pressure of the low pressure pumps is less than 20 bars.
In the case of a serious breach in the primary circuit of the reactor, the injection circuits and the medium pressure accumulator come into play very rapidly to introduce large amounts of water into the primary circuit to avoid the degradation of the fuel assemblies constituting the core of the reactor under the effect of too considerable heating and to maintain a possibility of subsequent cooling of the core by circulation of a cooling liquid.
The intervention of the accumulators is however limited to a very short period following the appearance of the rupture in the primary circuit. This period may be less than one minute.
During this very short time, the primary circuit has passed from the rated operating pressure, that is to say 155 bars to a low pressure at the order of some bars.
However, the core of the reactor and the whole of the vessel which contains it are still at high temperature, so that it is necessary to maintain the low pressure injection circuit in operation for a time which may be rather long to produce the cooling of the reactor.
It is hence necessary to use at least two different pump injection systems at different pressures arranged outside of the safety enclosure of the reactor.
This equipment used for the safety of the reactor must comply with very strict conditions as regards its design and, in particular, each of the systems is dual.
It is obviously possible to use the volumetric control circuit to ensure the high pressure injection in the case of rupture in the primary circuit but a quite special design of the volumetric control circuit is then necessary in order that it may be able to comply with the conditions required for a safety system.
In all cases, the investment necessary for the safety injection circuits and the complexity of these circuits are very considerable.
On the other hand, the low pressure injection circuit contributes to ensuring, as soon as the pressure of the primary circuit has fallen to a low value, the filling of the vessel with cooling water, and in particular the reimmersion of the core, that is to say the re-establishment of complete immersion of the latter in the water and its subsequent cooling by the flow of water. These extremely important functions must be effected by a safety circuit which is to be found outside the enclosure of the reactor and in consequence, the design of this circuit must be provided so as to obtain extremely good operating safety.
It has also been proposed to use automatic tripping safety systems arranged inside the safety enclosure of the reactor, but these devices are not capable of complying with all cases of accidents, in particular in the case of breaches of considerable size in a cold branch of the primary circuit. It is necessary to join with the automatic tripping devices, active equipment such as pumping means possibly integrated into the enclosure of the reactor.